First off, the ryze nerf was quite significant,I'll give riot that. 15 damage off his Q shaves off about 60 damage (Q-E-Q-W-Q-E-Q, s 4x15). However, Ryze is still beyond frustrating to fight against.
The change to fixing him would be simple. Make it so his E doesn't spread to other minions when cast. Revert it so if ryze uses his E on a spell flux'd enemy, spell flux spreads. This gives some time for the enemy to respect ryze is gonna clear the wave, and gives them 2 seconds to make a move. Really sucks when you're behind minions and his Q still hits you.
With this minor change, his damage output would be the same, but with more E usage, his mana would deplete a bit more and it would stall his waveclear